Kurdish fighters and their allies have captured a town once held by Islamic State (IS), bringing them closer to the extremist group's main stronghold in Syria.

The capture of the town of Ein Issa - hours after the Kurdish troops took the nearby Brigade 93 military base - increases the pressure on IS several days after they lost a major supply line when the Kurds captured the town of Tal Abyad on the Turkish border.

The advance put the Kurdish guerrillas within about 30 miles of the IS stronghold Raqqa. But even with the aid of US-led air strikes, battling close to Raqqa could prove costly for the Kurds and allied Syrian rebel factions.

The Britain-based Syrian Observatory for Human Rights and Kurdish activist Mustafa Bali said Kurdish fighters from the People's Protection Units, or YPG, captured the military base on Monday night.

The Observatory and YPG spokesman Redur Khalil said Kurdish fighters and their allies took over the nearby town of Ein Issa, the last major residential area north of Raqqa, which IS considers the capital of its self-declared "caliphate" or Islamic state that spans across Syria and Iraq.

Mr Khali said: "Ein Issa and dozens of villages around it are under our control."

He said the next task is to enforce and protect these areas because they know IS will strike back.

The Observatory said the YPG and its allies are also trying to gain control of a highway linking the northern city of Aleppo, Syria's largest, with the north-eastern city of Hassakeh.

The YPG's official Facebook page said "dozens of Daesh (IS) mercenaries were killed" at the Brigade 93 base. The Observatory said IS militants transferred the corpses of 26 of its fighters to Raqqa after they were killed in Ein Issa by air strikes.

"Operations will continue, but it is imperative that we first attempt to secure areas under our control," said Nawaf Khalil, head of the Germany-based Kurdish Center for Studies.

"Raqqa is a vast area and attacking it will need a great deal of coordination with other groups and the international alliance."

The US has found a reliable partner in the YPG, which has been the main force in the battle against IS in Syria.

The Kurds are moderate, mostly secular fighters, driven by revolutionary fervour and deep conviction in their cause. They are backed by Arab tribesmen, Assyrian Christian gunmen and members of the rebel faction known as Burkan al-Furat - Arabic for the "Volcano of the Euphrates".

It remains unclear whether the Kurds will try and push farther toward Raqqa.

Also, despite the recent Kurdish gains, IS militants still have another supply line from Turkey, this one running through north-western Syria to Raqqa.

When cornered in the past, the militants have relied on coordinated mass suicide car bomb attacks and other scorched-earth tactics.

Those tactics have included mass killings. On Tuesday, a media arm of IS in Iraq posted a video online purporting to show it kill over a dozen men it described as spies by drowning them in a cage, decapitating them with explosives and firing a rocket-propelled grenade at them in a car.

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